1.Field of the Invention
This invention relates to the stabilization of proteases in built, anionic rich aqueous detergent compositions.
2. Prior Art
The use of proteases in heavy duty liquid detergent formulations (HDLs) is complicated by their limited stability in solution. Two processes which limit the shelf life of a protease in a liquid are denaturation and autolysis (self-digestion). Denaturation of proteases may be minimized by selection of formulation components (i.e., actives, builders, pH etc.) so that acceptable enzyme stability can be achieved. Self digestion of proteases may be minimized by inclusion of a protease inhibitor. The inhibitor is released from the enzyme upon dilution in the wash.
Various protease inhibitors are known in the art. Hora et al. U.S. Pat. No. 4,261,868 teaches the use of borax as a protease inhibitor and both Shaer U.S. Pat. No. 4,243,546 and British Patent No. 1,354,761 (Henkel) teach the use of carboxylic acids as protease inhibitors. Various combinations of these protease inhibitors are also known in the art. Kaminsky et al. U.S. Pat. No. 4,305,837, for example, teaches the combination of carboxylic acids and simple alcohols and Tai U.S. Pat. No. 4,404,115 teaches the combination of borax and polyols as protease inhibitors. Severson U.S. Pat. No. 4,537,707 teaches the combination of borax and carboxylates as protease inhibitors.
As mentioned above, the use of carboxylates in detergent compositions as protease inhibitors is known. Letton et al U.S. Pat. No. 4,318,818, for example, teaches stabilized, liquid enzyme compositions in which the inhibitor is a short chain length carboxylic acid salt selected from the group consisting of formates, acetates, propionates and mixtures thereof. This patent teaches that formates are surprisingly much more effective than other short chain salts such as acetates and propionates. The reference also teaches that at a pH range above 8.5, only formates can be used. The detergent compositions used in this patent are unbuilt, i.e., contain no builders.
Shaer U.S. Pat. No. 4,243,546 teaches aqueous enzyme compositions wherein the enzyme stabilizer is selected from the group consisting of mono and diacids having from 1 to 18 carbon atoms. Acetic acid is said to be preferred. Compositions of the invention are also unbuilt. The patent seems to be primarily directed to compositions having a pH below 8 (most of the examples have a pH of 7.5) and the one example which has a pH of 9.5 appears to require the presence of alcohol (ethanol). In addition, the compositions of Shaer not only are not anionic rich, but appear to comprise no anionics at all.
British Patent 1,354,761 (Henkel) teaches compositions which may contain 2 to 8 carbon carboxylic acids. All the examples show use of acetic acid and the detergent compositions of the invention are also unbuilt.
Thus, where carboxylic acid stabilizers are used in the prior art, there is a preference for 1 or 2 carbon carboxylic acids (acetate and formate). When compositions of high pH (i.e. greater than 8.5) are used in the prior art, either the use of formate is dictated (as in Letton et al. U.S. Pat. No. 4,318,818) or the carboxylic acid is used in combination with an alcohol or in an environment which is not anionic rich. The compositions of the prior art are also unbuilt and there appears to be n recognition of the importance of using anionic rich compositions with specific stabilizers.
Unexpectedly, applicants have discovered that, when the detergent composition is a built, anionic rich composition having a pH greater than 8.5, more preferably 9.0 and above, enzyme stability is enhanced relative to other carboxylic acid stabilizers (i.e. acetate or formate) by the use of propionate rather than acetate or formate.